In conventional electrical transmission systems, the use of a signal bus is often desirable, in particular for local transmission with many connecting points. For this purpose, the individual transmitter and receiver stations are connected to a single transmission channel, the bus. The number of connectable units is not determined by the structure of the transmission path but only by its capacity to carry data in electrical form. As a result, signal bus systems offer a high degree of flexibility with regard to future changes or extensions. For many applications it is expedient to design the bus as a closed ring or loop.
In electrical systems the bus can be operated either in a time multiplex mode or a frequency multiplex mode. In the former mode, the bus is assigned to each connected unit during a particular time slot. In the latter mode, each connected unit transmits information to or receives information from the bus at allocated frequencies.
In the field of optical data transmission, a light wave guide with its very high transmission capacity is basically suitable as a bus, since there can be a great number of broad-band single channels. Owing to the limited modulation capability of known light transmitters and receivers (up to about 10.sup.8 cps), the frequency multiplex method would be advantageous for light wave guides.
However, if it is to be realized, an optical broad-band communication system with frequency coding requires couplers (for connections, branches, taps, etc.) as well as optical frequency coders of maximum bandwidth. These are not yet available.
Although an optical transmission channel with light guides and frequency multiplexing has been proposed (German Offenlegungsschrift No. 25 01 791), the transmitter and receiver are at the end of the optical fiber instead of being distributed at arbitrary connecting points along said fiber, as would be necessary for a true bus system.
The known optical fiber transmission systems operate in accordance with a single channel principle, using only one wavelength. Direct connections via optical fibers are arranged between transmitter and receiver or operations proceed in a time multiplex mode. U.S. Pat. No. 3,953,727 discloses an example of an optical time-multiplexed system. In such systems, the greatest part of the theoretical bandwidth of the optical transmission medium remains unutilized.
To provide a modular optical communication system which can be realized in practice, the following requirements have to be met:
(1) Simple coupling to the optical bus.
The connecting points for the individual connectable units must be arbitrarily definable along the light wave guide. The elements necessary for coupling the light in and out of the bus must be as simple, small and light as possible.
(2) Independence of different transmissions.
Data transmission in a particular wavelength range must be possible between two or several connections without interfering with other transmissions.
(3) Modularity.
The individual components must be strictly modular and be exchangeable without alignment.
(4) Directionality.
Transmission must be possible in both directions; couplers with and without directionality are required.
(5) Fail Safeness.
In a system with a great number of connections, care must be taken that failure of individual components does not affect the system. For example, the introduction of a new connection must not interfere with normal operation. In the case of maintenance work, it should be possible to bridge partial segments of the transmission path without adversely affecting the remainder of the bus.
(6) Low losses.
All optical components used must have very low optical losses to permit great distances between the refresh amplifiers. Connections which are temporarily not occupied by a device must not degrade transmission.